Yes as soon as the monstrosity comes out the boss takes 99% less damage until you kill it... their tactic is keeping dmg off the boss completely (aside from the constructs stacking the debuff) so they can build as many stacks as possible, and just bursting it down at ~70%... they also time it with both of his casts to get more time (about an extra 15 seconds)

When we were first trying to kill the boss on heroic we were trying the exact same tactic but we figured out this doesn't really matter since the "normal" way to kill it seems easier:

Just keep one of your tanks in the construct from ~72% until the fight ends and build stacks of the debuff... I'm pretty sure our tank was able to get a few stacks on the monstrosity as well (when other people get transformed they focus on building stacks on the monstrosity & interrupting it but your tanks main job is keeping stacks on the boss). After a few minutes the monstrosity should die and you can lust in phase 3 (the boss should have a ton of stacks at this point) and just melt the boss without even worrying about the phase 3 mechanic.

We haven't tried holding off dps in phase 1 with this tactic, since this way seems much easier and you don't have to worry about enrage

boss goes immune if u are too fast and push him under 70 percent at the same time as he starts casting scalpel.. he (or it?) just has to start cast scalpel above 70 and everything goes the same way as u saw in video above

There are some tactics talking about pushing the boss fast in P2 then having the debuff fall off and nuking the add to 3-5%. At that point you rebuild the stacks on the boss to about 20 before killing the add. Is this tactic doable or will we fail due to the fact that there are no puddles to eat in P3?